For what it's worth here's the full paragraph you quoted from:
Creating new languages is fun. Of course, people started making new
languages without understanding their history and reintroduced old mistakes. For example, many consider the handling of whitespace (the spaces
between words) in the Ruby language to be a replay of a mistake in the
original C language that was fixed long ago. (Note that one of the classic
ways to deal with a mistake is to call it a feature.)
The Secret Life of Programs by Jonathan E. Steinhart page 228
Which still leaves room for interpretation. So for an authoritative answer you'd have to ask Steinhart. To me this reads like Steinhart just doesn't like languages that ignore whitespace, which C and Ruby do. Others like Python don't, thus fixing the "mistake".
It's easier to read it this way when you know this about those languages off the top of your head. Authors who do can forget that not everyone does.
That's my guess. But I'm not Steinhart. So for all I know this refers to some weird whitespace handling bug that showed up in C, was fixed in C, and then showed up in Ruby. If that's what this is, it's really annoying that the bug isn't specifically called out. Since the text isn't clear you'd have to ask the author.